or 
(25)
Substituting for (Px) from equation (25) in equation (24),
(26)
Let
w = 
Then
or
Furthermore,
when x=0, then 
and when w = L, then 
.
Substituting
for
and (dx) in equation (26)
and inserting the new limits and integrating,
or
Thus,
(27)
Thus, the complete HETP equation for a packed GC column, that takes into account the compressibility of the carrier gas, will be
(28)
